Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on October 01, 2016, 04:25:16 pm
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I recently purchased a Gibson GA-18T amp. When I got it, the trem didn't work. I rebuilt it using this schematic:
http://www.davidsonamp.com/sg/gschemGGA16t.jpeg (http://www.davidsonamp.com/sg/gschemGGA16t.jpeg)
(GA-18T and 16-T use the same trem circuit)
I replaced the .3, .03, and two .01 caps, as well as the 68K, 510K, and the 2.2M resistors. I also replaced both pots using correct reverse audio values. I then replaced the 1K cathode resistor on V2B. The cathode cap was already changed to a modern 25uf before I received it. Still looks fine, and tests good with low resistance on my ESR meter.
So the trem works, but behaves a little funny. The "depth" knob doesn't kick in until about a quarter turn, and when it does, it's already pretty choppy. The depth is audibly changed through the next quarter turn, but the remaining half turn doesn't seem to produce much of an audible difference. Maybe a little, but hard to tell.
The frequency is also too fast. At zero, it 's got a nice pulse, but is still a bit fast for my liking. By half turn, it's an absurdly fast stutter, and doesn't seem to increase all that much through the rest of the turn.
Is this normal behavior? How can I slow down the speed and make the depth more usable? Any specific components worth checking out?
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Some good info here http://el34world.com/Forum/index.php?topic=20911.0 (http://el34world.com/Forum/index.php?topic=20911.0)
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I'm not able to open the link
http://www.davidsonamp.com/sg/gschemGGA16t.jpeg (http://www.davidsonamp.com/sg/gschemGGA16t.jpeg)
however here is the schematic of the GA-18T
(http://i.imgur.com/mkE58rr.jpg)
Franco
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the 18-T had several versions with various parts substituted. Mine uses 12AX7's and 6V6's, and the trem circuit does indeed match the 16-T with slightly different values than the "official" 18-T schematic
here's the 16-T
(http://rudn.nodevice.com/preview/big/365/365453-1.jpg)
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Did you "rebuild" it just because the trem didn't work?
Check the actual taper of your "reverse audio" pots. In this case, it can be done in-circuit. Trem switch ON. Wiper to ground should be:
Knob "0" - low-low R
Knob "5" - 1/10th value
Knob "0" - full value
The "1/10th" may be 1/5th on modern pots. That's not wrong, though not like old pots. Some modern pot makers don't have a clue and their "reverse audio" are anything but.
Choppy-- what is the bias (cathode voltage) on the 6V6es?
The speed should be 17Hz at somewhat off the slowest rate. This is indeed fast for some genres. You see the 0.03u 0.01u 0.01u string? Double or triple all three caps to get 1/2 or 1/3rd the rate. You may also need to increase the 20u cathode cap (go for 50uFd, or a Red LED).
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Did you "rebuild" it just because the trem didn't work?
Based on some research, a lot of folks have trouble with the trem in these amps. The resistors drift, caps go bad, etc. My amp is in rough physical shape, the original speaker is long gone, and I'm planning on keeping it to gig/record, so I'm not terribly concerned with preserving original parts. I had the parts on hand, so I tossed 'em in.
Check the actual taper of your "reverse audio" pots. In this case, it can be done in-circuit. Trem switch ON. Wiper to ground should be:
Knob "0" - low-low R
Knob "5" - 1/10th value
Knob "0" - full value
depth knob wiper to ground: 0=low R half=225k full=250k. Is my pot wired backwards???
Choppy-- what is the bias (cathode voltage) on the 6V6es?
19V on each cathode. Shared resistor is a modern 250 ohm 5w, and the cap is a modern 20uf 500v. JJ 6V6's installed.
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Have you seen this page? I think some of the tremolo values were modded for better range and depth.
http://djlservice.com/AudioGibsonGA18wPhotos.htm (http://djlservice.com/AudioGibsonGA18wPhotos.htm)
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Have you seen this page? I think some of the tremolo values were modded for better range and depth.
http://djlservice.com/AudioGibsonGA18wPhotos.htm (http://djlservice.com/AudioGibsonGA18wPhotos.htm)
indeed. I went ahead and changed the .01 caps to .02 and the speed is much more usable. I originally upped the .03 to .06 but that put the brakes on it a little too much. I'm satisfied with .03, .02, .02
I still have a problem with the sweep of the depth knob. It does nothing until a quarter turn, then I can control the depth between quarter turn and half turn. Half turn to full turn doesn't do much.
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> Is my pot wired backwards???
Smells like Audio, not Reverse Audio.
"Wire it backward" (swap the two outside leads), TURN it backward (ten to zero), and see if it feels better except wrong-direction. Then get a real REVerse Audio pot.